1. Field of the Invention
This invention relates to the field of digital communications, and in particular to a method of implementing turbo trellis code modulation in a flexible manner that can be adapted to transmission conditions.
2. Description of Related Art
In a digital communications channel it is common practise to include some form of coding scheme to increase the data throughput. Recently, turbo codes have gained in poplularity due to their large coding gains. See Berrou and A. Glavieux, “Near Optimum Error Correcting Coding and Decoding: Turbo-Codes”, IEEE Trans. on Communications, Vol. 44, No. 10, October, 1996. A turbo coder is a combination of two simple encoders. The input is a block of K information bits. The two encoders generate symbols from two simple recursive convolutional codes, each with a small number of states. The information bits are also sent uncoded. A key feature of turbo codes is the interleaver, which permutes the original K information bits before input to the second encoder. The permutation ensures that input sequences for which one encoder produces low-weight codewords generally causes the other encoder to produce high-weight codewords. Thus, even though the constituent codes are individually weak, the combination is powerful.
Turbo codes have been applied in DMT (Discrete Multitone) systems, for example, used in xDSL transmission. In xDSL systems, turbo code can be used to replace other types of trellis code to get better Bit-Error Rate(BER) performance. See, Hamid R. Sadjadpour, “Application of Turbo Codes for Discrete Multi-Tone Modulation Schemes”, AT&T Shannon Labs., 1999. However, when the constellation size increases, the coding gain advantage of turbo code starts to diminish. This is because the redundant bits make the constellation size even larger. Turbo-trellis coded modulation, in which only the least significant bit (LSB) in constellation is coded has been introduced to achieve better performance than other trellis-coded modulation.
DMT is a type of Multicarrier Modulation. The basic idea behind multicarrier modulation is that multiple channels can be established with digital signal processing techniques using the Fast Fourier Transform (FFT). A DMT modem encodes bits into symbols and sends them through an inverse FFT and then converters the digital signal into analog, to send it through the copper phone wires. The receiving modem reverses the process. Many subchannels are used to transmit data, each having a different carrier and a different QAM (Quadrature amplitude modulation) constellation containing different number of bits per constellation. The multiple of carriers are implemented through the Discrete Fourier Transform. The number of data being transmitted per DMT symbol varies from 16 bits per symbol for ADSL upstream data transmission to a maximum 15000 bits per symbol for VDSL system. Since turbo code works well with a large interleaver size (typically larger than 1000 bits), at low data rates more bits, and possibly all the bits, need to be transmitted transmission to meet the latency requirement of the system. At high date rates, it is too costly to code all the data. Also, the performance will suffer for high constellation if all data are encoded.
There is a need for an effective encoder suitable for DMT applications.